1. Field of the Invention
The invention generally relates to surface-emitting laser elements, surface-emitting laser arrays, optical scanner devices and image forming apparatuses, and more particularly to a surface-emitting laser element capable of emitting a laser beam in a direction perpendicular to a substrate, a surface-emitting laser array including therein plural of such surface-emitting laser elements, an optical scanner device having such a surface-emitting laser array, and an image forming apparatus having such an optical scanner device.
2. Description of the Related Art
Numerous studies have been carried out on a surface-emitting laser element (i.e., a surface-emitting semiconductor laser element) capable of causing laser oscillation in a direction perpendicular to a substrate. The surface-emitting laser element includes a low threshold current for oscillation to emit a laser beam having a high-quality circular outgoing beam shape compared to an edge emitting semiconductor element. In addition, since the surface-emitting laser element is capable of emitting a laser beam in a direction perpendicular to a substrate, it is easy to integrate laser beams two dimensionally with high density. Accordingly, applications of the surface-emitting laser elements to a light source for a parallel optical interconnection or a high-speed and high accuracy electrophotographic system have been examined.
The surface-emitting laser element generally includes a constricting structure to enhance current entry efficiency. Such a constricting structure is an Al (aluminum) selective oxide constricting structure (hereinafter also called an “oxide constricting structure” for convenience). Examples of the oxide constricting structure are disclosed in Applied Physics Letters, vol. 66, No. 25, pp. 3413-3415, 1995 (K. D. Choquette, K. L. Lear, R. P. Schneider, Jr., K. M. Geib, “Cavity characteristics of selectively oxidized vertical-cavity lasers”, Applied Physics Letters, vol. 66, No. 25, pp. 3413-3415, 1995: also referred to as “Non-Patent Document 1”), and Electronics Letters, No. 24, Vol. 30, pp. 2043-2044, 1994 (K. D. Choquette, R. P. Schneider, Jr., K. L. Lear, K. M. Geib, “Low threshold voltage vertical-cavity lasers fabricated by selective oxidation”, Electronics Letters, No. 24, Vol. 30, pp. 2043-2044, 1994: also referred to as “Non-patent Document 2”).
Further, a laser printer introducing 780 nm VCSEL arrays (surface-emitting laser arrays) is disclosed in Electronic Components and Technology Conference Proceedings, Vol. 2, 2004, pp. 1371-1375 (H. Nakayama, T. Nakamura, M. Funada, Y. Ohashi, M. Kato, “780 nm VCSELs for Home Networks and Printers”, Electronic Components and Technology Conference Proceedings, 54th, Vol. 2, June, 2004, pp. 1371-1375: also referred to as “Non-Patent Document 3”).
Japanese Patent Application Publication No. 11-48520 (hereinafter referred to as “Patent Document 1”) discloses an image forming apparatus having a multi-beam light source.
In applying surface-emitting laser elements to image forming apparatuses such as printers, a small spot size of a beam is preferable to converge on a photoreceptor. Further, since the reflectance and the transmittance of an optical system are polarization-dependent, polarization of a beam may need to be aligned in a specific direction in order to prevent change in laser intensity on the photoreceptor. Moreover, high laser output power may be preferable in carrying out fast writing (recording). That is, in the application of the surface-emitting laser elements to the image forming apparatus, it may be necessary to acquire high output power in a single basic transverse mode (i.e., single mode) operation, and align the polarization direction of a beam in specific direction. Note that in other applications of the surface-emitting laser elements, it is preferable that high single mode output power be obtained and the polarization direction be aligned in a specific direction. Thus, extensive studies have been carried out on the enhancement of single mode output and stabilizing the polarization of a beam.
Japanese Patent Application Publication No. 2001-156395 (hereinafter referred to as “Patent Document 2”) discloses a surface emission semiconductor laser element in which a layer structure of a semiconductor material is formed on a substrate. The layer structure of the semiconductor material includes upper and lower reflector layer structures and an emission layer sandwiched between the upper and lower reflector layer structures. An opening located above the upper reflector layer structure is coated with upper electrode layers having a circular plan view, that are transparent for the oscillation wavelength of laser light.
Japanese Patent No. 3955925 (hereinafter referred to as “Patent Document 3”) discloses a vertical cavity surface emitting laser device having an opening portion and a semiconductor discontinuity portion formed within the body of the device at a position distant from an edge of the opening portion. The semiconductor discontinuity portion is formed of a slit that is filled with a material differing from the semiconductor material, and a side wall of the semiconductor discontinuity portion has an opening of the slit formed such that the side wall is extended in a desirable direction of polarization of laser light emitted from the laser device that is substantially aligned with a boundary of the discontinuity portion.
Further, Japanese Patent Application Publication No. 2007-201398 (hereinafter referred to as “Patent Document 4”) discloses a surface-emitting semiconductor laser element that includes a substrate, a first multilayer reflection film on the substrate, an active layer having a light emitting central region formed on the first multilayer reflection film, a second multilayer reflection film formed on the active layer having the light emitting central region, and a laser transverse mode adjusting layer formed on the second multilayer reflection film. At least one of the first multilayer reflection film and the second multilayer reflection film includes a quadrilateral current injection region having its intersection of diagonal lines disposed at the light emitting central region of the active layer, and the second multilayer reflection film includes a light emitting window provided in a region corresponding to one of the diagonal lines of the current injection region, and a pair of grooves located at both sides of the light emitting window. The laser transverse mode adjusting layer is provided corresponding to the light emitting window and having a peripheral region excluding the central region of the light emitting window having a reflection factor lower than that of a region of the light emitting window corresponding to the light emitting central region of the active layer.
Japanese Patent Application Publication No. 2004-289033 (hereinafter referred to as “Patent Document 5”) discloses a surface-emitting semiconductor laser element that includes a first multilayer reflection film, an active layer formed on the first multilayer reflection film, and a second multilayer reflection film formed on the active layer. At least one layer of the first multilayer reflection film and the second multilayer reflection film includes a first region disposed at least at the part of a region corresponding to a part of the active layer, and having a thickness of substantially λ/4n (λ: oscillation wavelength, n: refractive index) and a second region disposed on a region excluding the first region and having a thickness of substantially excepting λ/4n.
However, although the vertical cavity surface emitting laser device disclosed in Patent Document 3 is capable of regulating the polarization direction, it may be difficult to suppress the oscillation of the high-order transverse mode of a laser beam due to a change in a laser light confinement effect in a transverse direction based on depths of grooves.
Further, in the surface-emitting semiconductor laser element disclosed in Patent Document 4, if the gap between the grooves is made narrower than the current confined region for regulating the polarization direction, a current passage region may be substantially reduced (narrowed). This has increased electric resistance or electric current density, thereby reducing the life-span of the laser element.
Moreover, in the surface-emitting semiconductor laser element disclosed in Patent Document 5, the growth of the crystal is temporarily stopped after allowing the crystal to grow up to a layer adjacent to the active layer. The crystals are then allowed to grow again after patterning of the resist and etching of the film are carried out. In this case, when the crystal growth is initiated again, the etched surface of the film may affect the crystal growth, which has brought variability in the characteristics of the laser element or in the control characteristics of the transverse mode of a laser beam. Thus, the laser element may not be suitable for mass production.
Applicants of the present application have conducted various experiments to examine the control of a high-order oscillation transverse mode of a laser beam and the regulation of a polarization direction of a laser beam in a desirable direction, and have found that a polarization mode suppression ratio PMSR may be lowered, in comparison to a case where the reflectance within the emission region is uniform, if a circular low reflectance portion is provided within the emission region as disclosed in the surface-emitting semiconductor laser element disclosed in Patent Document 2. Note that the polarization mode suppression ratio PMSR is a ratio of light intensity of a desired polarization direction of laser light to light intensity of a direction perpendicular to the desired polarization direction of laser light.
Even if the surface-emitting laser element is capable of controlling the polarization direction in one direction based on the gain anisotropy of the active layer obtained by providing the slanted substrate, the polarization direction may become unstable by providing the circular low reflectance portion in the emission region.